problems with original wad 300b

[Richard]

really

[Clive]

Well that's the difference between making a diy amp that works and a production product that lasts.

11W will be fine, for the few amps out there as there will be only a few failures and as it's diy they can be fixed relatively easily. There will be little of no publicity either so it's your choice.

Personally I'd use 2 x 3k 11W in parallel.

[Ianm2]

if you want to guarantee it will virtually never fail, I would go for the goldish bodied 50 watters, and glue them with heatsink compound to the bottom of the chassis. that's the proper way to do it!!

[Clive]

Originally Posted by Ianm2

if you want to guarantee it will virtually never fail, I would go for the goldish bodied 50 watters, and glue them with heatsink compound to the bottom of the chassis. that's the proper way to do it!!

This is similar to what diyhifisupply do. They used 25W gold metal body resistors for the Bille amps, they now use 30W wirewound for the LD 91. They don't want their kits getting a bad name due to component failures.

A 3 x rating is usyally believed to be sufficient but more won't hurt!

Richard, remember you're asking an 11W resistor to continually burn 7W, of course it will work but don't be surprised if it fails after 10 years. How well it copes will also depend on ventilation and the ambient temp under the chassis.

[Richard]

A lot of things will fail after 10 years Clive It's about risk and you're looking at the wrong parts.

Even if it did fail the resistor will not usually cause a problem. It would be more sensible (if you are concerned) to triple the voltage rating of the cathode bypass caps.

Some of these over-rating "rules of thumb" (whos?) may come from components such as rectifier diodes and their smoothing caps in which the ratings are not immediately obvious due to intermittent rather than continuous operation. Other reasons might be for the subjective "sound quality" or genuine but tiny "noise" reasons. Other reasons might be harsh conditions such as heat or humidity.

In the case of these cathode resistors though it's hard to imagine more docile conditions; passing steady current well within rating and rated to run up to 250C in a pleasant ventilated environment - what a cushy job

Rich

[Clive]

I said 10 years because it seemed relevant to this particular failure but it could be 2 years, who knows...

Looking at the Meggitt 11W specs:

Load life is 1000 hours at 70C, I assume this means life if used at the max 11W. Maybe at 7W it's 3,000 or 8,000 hours? Either way that's not enough.

At 7W the graph shows they operate at 180C. 7W continuous is not that cushy for an 11W max resistor.

It just seems like an unnecessary risk that for a few pence can be managed rather better.

[Richard]

A higher rated resistor might last longer - but - the correctly rated one will last a long time - may simply never fail - and the money and concern may be better spent elsewhere such as long-life caps for the bypass.

Use a higher rating if it's convenient but these are the original rating and I see no reason for worry.

For practical examples look at Quad and Mullard. The single 180R 3W in a QuadII dissipates 3.74W. Everyone knows the resistor is under-rated but I've had 3 pairs over the years and none has ever failed. What DOES fail is the cathode bypass cap, when they get old, and then you get the same experience as Julian.

I have had that happen but I've never had a WW cathode resistor fail in 30 years valve amp use. I also use 3W cathode Rs (as specified by Mullard) in my 5-20s which run at 2.2W dissipation.

Rich

[Richard]

My thoughts about the QuadII resistor rating btw, it seems strange that Quad would get this wrong, Morgan Jones even calls it curious....

I wonder if, as it's a low-risk failure, Quad deliberately rated it on/over the limit to act as protection if the KT66s started to draw too much current. That is; better to have a repair for a cathode resistor than need a new OPTX.

This would be in line with the practice of Leak who used a similarly marginally rated WW in their ST20 power supply. It was designed to heat the solder and drop off, thus disconnecting, if the current draw was too much.

Rich

edit;

Yes, of course, just looked at the QuadII circuit and the unusual OPTX design with the cathode connection to ground made through a TX winding. If the KT66s drew too much, say, due to a leaking coupling cap, the TX may be overheated.

Rich

[NealG]

Julian, I use 2K2 and 3K3 in parallel in this amp. to give 1K32, they are non-inductive RS-2392733 and 2392727. Rated at 10W each. The 2K2 dissipates 4W and the 3K3 2.75W.

The original uses a 1K5 11W in parallel with 6K8 3W. The 1K5 dissiapates 6W and the 6K8 1.33W.

I've not had a WW cathode R fail either but I'm also a bit belt 'n' braces so like to overspec, hence the RS resistors.

I would take a close look at the caps. as Rich suggests, IMHO they need to be 105 deg C rated for long life

I was working on this amp the other weekend and was a little concerned when after an hours use and back on the bench I put a finger on the top of one Cathode Cap and promptly removed it! From experience that normally indicates the temp is close to 70 deg C... and they are 85 deg C rated black gates.....

[Richard]

Originally Posted by NealG

I would take a close look at the caps. as Rich suggests, IMHO they need to be 105 deg C rated for long life

I was working on this amp the other weekend and was a little concerned when after an hours use and back on the bench I put a finger on the top of one Cathode Cap and promptly removed it! From experience that normally indicates the temp is close to 70 deg C... and they are 85 deg C rated black gates.....

Hi Neal, yes indeed, the resistors are directly under large vent slots fed from below but the caps are tucked away and will get hot. Not measured mine but I did probe Kit88 when I had it; although it felt hot the abient temp was 48C.

Rich